Advanced Television System Committee (ATSC) Standards for High Definition Television (HDTV) data transmission through a terrestrial broadcasting channel utilizes signals obtained by modulating 12 independent data streams that have gone through trellis encoding and time-multiplexing into 8-VSB symbol streams {−7, −5, −3, −1, 1, 3, 5, 7} of 10.76 MHz rate. The signals are converted into signals of a 6-MHz frequency band, which correspond to standard Very High Frequency (VHF) or Ultrahigh Frequency (UHF) terrestrial television channels. Signals of the channel are broadcasted in a data rate of 19.39 Mbps. More details for the ATSC DTV Standards and A/53 are available at http://www.atsc.org/.
FIG. 1 is a block diagram illustrating a typical Digital Television (DTV) transmitter. The typical DTV transmitter includes a data randomizer 101, a Read Solomon (RS) encoder 103, a data interleaver 105, a trellis encoder 107, a multiplexer (MUX) 109, a pilot adder 11, a Vestigial Side Band (VSB) modulator 113, and a radio frequency (RF) converter.
Data inputted to the transmitter 100 are serial data streams each formed of 188-byte Moving Picture Experts Group (MPEG)-compatible data packets, the 188-byte data packets including one synchronization byte and 187 bytes for payload data.
The inputted data are randomized in a data randomizer 101. Each packet is encoded to include 20-byte parity information for Forward Error Correction (FEC) in the RS encoder 103. The FEC includes RS coding, ⅙ data field interleaving, and ⅔ trellis coding. According to the ATSC Standards, the data randomizer 101 performs XOR on all payload data bytes inputted to pseudo random binary sequences (PRBS) each of which has a maximum length of 16 bytes and initialized at a data field beginning point.
The RS encoder 103 receives randomized data outputted from the data randomizer 101. It generates data having a total of 207 bytes to be transmitted for each data segment by adding 20 RS parity bytes for FEC to 187-byte data. The randomizing and FEC process are not performed on the synchronization bytes corresponding to segment synchronization signals among the input packet data.
Subsequently, data packets included in a consecutive segment of each data field are interleaved in the data interleaver 105 to obtain interleaved data packets. The interleaved data packets are interleaved again in the trellis encoder 107 and then encoded.
The trellis encoder 107 generates data symbol streams each expressed in three bits by using inputs of two bits. One bit out of the two-bit input is precoded, and the other bit is 4-state-trellis-encoded to thereby output two bits. The three bits outputted from the trellis encoder 107 are mapped to an 8-level symbol. The conventional trellis encoder 107 includes 12 parallel trellis encoders and precoders to generate 12 interleaved and/or coded data sequences.
The 8-level symbol is generated as a data frame for transmission after combined in the multiplexer 109 with segment synchronization and field synchronization bit sequences 117 that are transmitted from a synchronization unit (not shown). Then, a pilot signal is added to the pilot adder 111.
The 8-level data symbol stream goes through a VSB suppressed-carrier modulation in the VSB modulator 113 to thereby obtain a base band 8-VSB symbol stream. The base band 8-VSB symbol stream is converted into RF signals in the RF converter 115 and then transmitted.
FIG. 2 is a block diagram showing a typical DTV receiver. The DTV receiver includes a tuner 201, an intermediate frequency (IF) filter and detector, a National Television Systems Committee (NTSC) rejection filter 205, an equalizer and phase tracker 207, a trellis decoder 209, a data deinterleaver 211, an RS decoder 213, a data derandomizer 217, and a synchronization and timing recovery block 215.
The RF signals transmitted from the DTV transmitter 100 are broadcasted through a channel selected by the tuner 210 of the DTV receiver 200. Then, the RF signals are filtered into IF signals and a synchronization frequency is detected in the IF filter and detector 203. In the synchronization and timing recovery unit 215, synchronization signals are detected and clock signals are restored.
Subsequently, the NTSC rejection filter 205 removes NTSC interference signals out of the signals through a comb filter, and the equalizer and phase tracker 250 performs equalization and phase tracking. The data symbols whose multipath interference is eliminated are encoded and go through trellis decoding in the trellis decoder 209.
The decoded data symbols are deinterleaved in the data deinterleaver 211. The deinterleaved data symbols go through RS decoding in the RS decoder 213 and derandomization in the data derandomizer 217. Accordingly, the MPEG-compatible data packets transmitted from the DTV transmitter 100 are restored.
FIG. 3 is a diagram depicting a transmitting data frame exchanged between the transmitter of FIG. 1 and the receiver of FIG. 2. As illustrated in the drawing, the transmitting data frame includes two data fields and each data field is formed of 313 data segments.
The first data segment of each data field is a data field synchronization signal, which is a synchronization signal, and it includes a data sequence for training, which is used in the DTV receiver 200. Each of the other 312 data segments includes 20-byte data for FEC of a 188-byte transport packet. The data in each data segment are formed of data included in a few transmitting packets for data interleaving. In short, the data of each data segment can correspond to a few transmitting packet data.
Each data segment is formed of 832 symbols. The first four symbols are binary and they provide data segment synchronization. A data segment synchronization signal corresponds to a synchronization byte, which is the first byte of 188 bytes that constitute an MPEG-compatible data packet.
The other 828 symbols correspond to the other 187 bytes of the MPEG-compatible data packet and the 20 bytes for FEC. The 828 symbols are transmitted in the form of 8-level signals. Each of the symbols is expressed in three bits. Therefore, data of a total of 2484 bits (2484 bits=828 symbols×3 bits) are transmitted for each data segment.
In the prior art, however, transmitting signals of a conventional 8-VSB transceiver are distorted in an indoor or mobile channel environment due to variable channel and/or multipath effects, which leads to degraded reception performance of the DTV receiver.
In short, transmitting data are under influence of various channel distortion factors, such as multipath effect, frequency offset, and phase jitter. To compensate for such signal distortion generated due to the channel distortion factors, a data sequence for training, which is to be referred to as training data sequence hereafter, is transmitted every 24.2 milliseconds.
However, multipath properties may be changed or Doppler Effect may occur to distort receiving signals even between the time intervals of 24.2 ms when the training data sequence is transmitted. What makes it worse is that an equalizer of the receiver does not provide a quick converging speed enough to compensate for the receiving signal distortion. So, the receiver can hardly carry out equalizing accurately. For this reason, the 8-VSB type transceiver shows lower DTV broadcasting reception performance than an analogue transceiver. Moreover, it cannot receive any signals in a mobile environment at all. Even if it receives signals, there is a problem that the signal-to-noise ratio (SNR) satisfying threshold of visibility (TOV) hikes.
To solve the above problems, another prior art is disclosed in International Publication Nos. WO 02/080559, WO 02/100026 and U.S. Patent Publication No. US2002/0194570, in which robust data are transmitted in the form of a 4-level symbol {−7, −5, 5, 7} or {−7, −3, 3, 7}.
The prior art restricts the symbols mapped with the robust data, so there is a problem that the average power of the symbols expressing the robust data is increased compared to the conventional technology utilizing an 8-VSB transceiver. The average power of robust data is 21 energy/symbol in the conventional 8-VSB transceiver.
That is, in case that the robust data are transmitted in the form of one among the 4-level symbols of {−7, −5, 5, 7}, the symbol average power is 37 energy/symbol. If the robust data are transmitted in the form of one among the 4-level symbols of {−7, −3, 3, 7}, the symbol average power is 29 energy/symbol. This shows increased average power of a symbol expressing robust data, compared to the conventional technology using an 8-VSB transceiver.
The increase in the average power of a symbol expressing robust data leads to the increase in the overall average power. If signals are transmitted with a limited power output, which is the usual case, the transmitting power of normal data is decreased relatively, compared to the conventional 8-VSB technology. Thus, there is a problem that the reception performance becomes worse than the conventional 8-VSB technology in the same channel environment.
This problem becomes more serious as the ratio of robust data mixed with normal data is increased. As a result, reception performance is degraded even in a good channel environment. Further, backward compatibility may not be provided to an 8-VSB receiver.